Gold ores are either free milling or refractory. Free-milling ores can be processed by gravity techniques or direct cyanidation. Refractory ores are not effectively processed by these techniques, and are more difficult to process since precious metals such as gold are often occluded or encapsulated within sulfide minerals, carbonaceous matter, or siliceous minerals of the ore. The distribution of gold within the ore can vary considerably from one ore to another. Refractory ores can be ores, flotation concentrates, mill tailings, and other reserves from mining operations.
In conventional processes, the carbon- and sulfur-containing components of refractory ores, such as organic and inorganic carbonaceous materials and sulfidic minerals, respectively, are oxidized during roasting or pressure-oxidative autoclaving of the refractory ore. The roasted or pressure-oxidized ore is subjected to cyanide leaching followed by a gold recovery process since gold combines with cyanide to form a complex. However, residual carbon (known as calcine) or clay materials such as illite, kaolin, and montmorillonite adsorb the gold-cyanide complex (a phenomenon known as “preg robbing”). This phenomenon can reduce precious metal recovery during cyanide leaching. Gold is extracted from the cyanide leach solution by conventional means (e.g., zinc dust precipitation process, carbon-in pulp process, carbon in-leach process, or flotation-roasting leaching).
After the ore is roasted or subjected to pressure-oxidative autoclaving, the ore is quenched with process water and a portion of the gold grains can become coated with gypsum. This can also occur during subsequent pH adjustment with lime or another alkali. Since gypsum is not susceptible to dissolution by alkaline cyanide, the gold grains that are “encapsulated” by gypsum are not solubilized during cyanide leaching and such gold is not recovered from the ore. It is believed that as much as 10% of the gold in refractory ore is unrecovered as a result of gypsum encapsulation.